Question: Why does my PC stop itself after working for 15 minutes or so? The CPU fan and the SMPS fan keeps on working.
Answer: If your computer is shutting down randomly but the power is still on it could indicate a faulty motherboard. If it were the power supply you wouldn't have the fans stay on. Many computers have a temperature sensor that will shut down the computer if it gets too hot but since you still have fans on its most likely not the case.
You should test to make sure it's not Windows related. You can boot the computer into the BIOS or to a Windows CD so that it stays running but doesn't go into Windows. You can even unplug the IDE cable from your hard drive so it can't load Windows and let it sit there and see if it shuts down.
Try booting into Safe Mode and use the computer for awhile and see if you get the same results. If not, then look for something that is loading with Windows that doesn't with Safe Mode. It may even be a video driver. Safe Mode uses a generic video driver so that may be a good test as well.